Internal Structure of Kuttara Caldera, Hokkaido, Japan

A controlled-source audio-frequency magnetotelluric (CSAMT) survey was conducted across the caldera of Kuttara volcano, Hokkaido, Japan, to investigate its subsurface structure. The caldera is 3 km in diameter and contains a circular lake (Lake Kuttara) 2.5 km across. The CSAMT survey was conducted along a 12-km-long east‒west-trending transect that crossed the volcano and passed over the caldera. A total of 23 receiver stations were distributed along the survey line, including 7 stations in the caldera. Unique on-boat measurements were obtained at the surface of Lake Kuttara. A two-dimensional inversion of the CSAMT data, which revealed the resistivity structure to depths of up to 1000m beneath the caldera, suggested the existence of a low-resistivity region (<30Ω・m) beneath the eastern caldera floor, extending subvertically for>1000m and with a width of 1300-1500m. The structure is interpreted to be a region filled with lava blocks and pyroclasts, which subsided during collapse of the caldera and which has been affected by hydrothermal alteration. The location of the low-resistivity region corresponds to an oval basin in the eastern part of the lake, implying that caldera subsidence occurred mainly beneath this basin. The western part of the caldera floor tilts gently to the east and has a rugged surface, suggesting that the western caldera rim was significantly enlarged as a result of landsliding during caldera formation. We thus infer that the Kuttara caldera was produced by asymmetric caldera collapse, which is attributed to the geological heterogeneity of the Kuttara volcano, with an eastern part comprised mainly of andesitic lavas and a western part comprised mainly of dacitic pyroclastic deposits.